2,244 research outputs found
Bubble size statistics during reionization from 21-cm tomography
The upcoming SKA1-Low radio interferometer will be sensitive enough to produce tomographic imaging data of the redshifted 21-cm signal from the Epoch of Reionization. Due to the non-Gaussian distribution of the signal, a power spectrum analysis alone will not provide a complete description of its properties. Here, we consider an additional metric which could be derived from tomographic imaging data, namely the bubble size distribution of ionized regions. We study three methods that have previously been used to characterize bubble size distributions in simulation data for the hydrogen ionization fraction – the spherical-average (SPA), mean-free-path (MFP) and friends-of-friends (FOF) methods – and apply them to simulated 21-cm data cubes. Our simulated data cubes have the (sensitivity-dictated) resolution expected for the SKA1-Low reionization experiment and we study the impact of both the light-cone (LC) and redshift space distortion (RSD) effects. To identify ionized regions in the 21-cm data we introduce a new, self-adjusting thresholding approach based on the K-Means algorithm. We find that the fraction of ionized cells identified in this way consistently falls below the mean volume-averaged ionized fraction. From a comparison of the three bubble size methods, we conclude that all three methods are useful, but that the MFP method performs best in terms of tracking the progress of reionization and separating different reionization scenarios. The LC effect is found to affect data spanning more than about 10 MHz in frequency (Δz ∼ 0.5). We find that RSDs only marginally affect the bubble size distributions
Dynamics of Primordial Black Hole Formation
We present a numerical investigation of the gravitational collapse of
horizon-size density fluctuations to primordial black holes (PBHs) during the
radiation-dominated phase of the Early Universe. The collapse dynamics of three
different families of initial perturbation shapes, imposed at the time of
horizon crossing, is computed. The perturbation threshold for black hole
formation, needed for estimations of the cosmological PBH mass function, is
found to be rather than the generally employed
, if is defined as \Delta M/\mh, the
relative excess mass within the initial horizon volume. In order to study the
accretion onto the newly formed black holes, we use a numerical scheme that
allows us to follow the evolution for long times after formation of the event
horizon. In general, small black holes (compared to the horizon mass at the
onset of the collapse) give rise to a fluid bounce that effectively shuts off
accretion onto the black hole, while large ones do not. In both cases, the
growth of the black hole mass owing to accretion is insignificant. Furthermore,
the scaling of black hole mass with distance from the formation threshold,
known to occur in near-critical gravitational collapse, is demonstrated to
apply to primordial black hole formation.Comment: 10 pages, 8 figures, revtex style, submitted to PR
General Analysis of Inflation in the Jordan frame Supergravity
We study various inflation models in the Jordan frame supergravity with a
logarithmic Kahler potential. We find that, in a class of inflation models
containing an additional singlet in the superpotential, three types of
inflation can be realized: the Higgs-type inflation, power-law inflation, and
chaotic inflation with/without a running kinetic term. The former two are
possible if the holomorphic function dominates over the non-holomorphic one in
the frame function, while the chaotic inflation occurs when both are
comparable. Interestingly, the fractional-power potential can be realized by
the running kinetic term. We also discuss the implication for the Higgs
inflation in supergravity.Comment: 16 pages, 1 figur
Cost of s-fold Decisions in Exact Maxwell-Boltzmann, Bose-Einstein and Fermi-Dirac Statistics
The exact forms of the degenerate Maxwell-Boltzmann (MB), Bose-Einstein (BE)
and Fermi-Dirac (FD) entropy functions, derived by Boltzmann's principle
without the Stirling approximation (Niven, Physics Letters A, 342(4) (2005)
286), are further examined. Firstly, an apparent paradox in quantisation
effects is resolved using the Laplace-Jaynes interpretation of probability. The
energy cost of learning that a system, distributed over s equiprobable states,
is in one such state (an s-fold decision) is then calculated for each
statistic. The analysis confirms that the cost depends on one's knowledge of
the number of entities N and (for BE and FD statistics) the degeneracy,
extending the findings of Niven (2005).Comment: 7 figures; 5 pages; REVTEX / TeXShop; paper from 2005 NEXT-Sigma-Ph
Higgs Chaotic Inflation in Standard Model and NMSSM
We construct a chaotic inflation model in which the Higgs fields play the
role of the inflaton in the standard model as well as in the singlet extension
of the supersymmetric standard model. The key idea is to allow a non-canonical
kinetic term for the Higgs field. The model is a realization of the recently
proposed running kinetic inflation, in which the coefficient of the kinetic
term grows as the inflaton field. The inflaton potential depends on the
structure of the Higgs kinetic term. For instance, the inflaton potential is
proportional to phi^2 and phi^{2/3} in the standard model and NMSSM,
respectively. It is also possible to have a flatter inflaton potential.Comment: 5 pages. v2:discussion and references adde
Quantum mechanical Carnot engine
A cyclic thermodynamic heat engine runs most efficiently if it is reversible.
Carnot constructed such a reversible heat engine by combining adiabatic and
isothermal processes for a system containing an ideal gas. Here, we present an
example of a cyclic engine based on a single quantum-mechanical particle
confined to a potential well. The efficiency of this engine is shown to equal
the Carnot efficiency because quantum dynamics is reversible. The quantum heat
engine has a cycle consisting of adiabatic and isothermal quantum processes
that are close analogues of the corresponding classical processes.Comment: 10 page
The Error is the Feature: how to Forecast Lightning using a Model Prediction Error
Despite the progress within the last decades, weather forecasting is still a
challenging and computationally expensive task. Current satellite-based
approaches to predict thunderstorms are usually based on the analysis of the
observed brightness temperatures in different spectral channels and emit a
warning if a critical threshold is reached. Recent progress in data science
however demonstrates that machine learning can be successfully applied to many
research fields in science, especially in areas dealing with large datasets. We
therefore present a new approach to the problem of predicting thunderstorms
based on machine learning. The core idea of our work is to use the error of
two-dimensional optical flow algorithms applied to images of meteorological
satellites as a feature for machine learning models. We interpret that optical
flow error as an indication of convection potentially leading to thunderstorms
and lightning. To factor in spatial proximity we use various manual convolution
steps. We also consider effects such as the time of day or the geographic
location. We train different tree classifier models as well as a neural network
to predict lightning within the next few hours (called nowcasting in
meteorology) based on these features. In our evaluation section we compare the
predictive power of the different models and the impact of different features
on the classification result. Our results show a high accuracy of 96% for
predictions over the next 15 minutes which slightly decreases with increasing
forecast period but still remains above 83% for forecasts of up to five hours.
The high false positive rate of nearly 6% however needs further investigation
to allow for an operational use of our approach.Comment: 10 pages, 7 figure
Einstein's "Zur Elektrodynamik..." (1905) Revisited, with Some Consequences
Einstein, in his "Zur Elektrodynamik bewegter Korper", gave a physical
(operational) meaning to "time" of a remote event in describing "motion" by
introducing the concept of "synchronous stationary clocks located at different
places". But with regard to "place" in describing motion, he assumed without
analysis the concept of a system of co-ordinates. In the present paper, we
propose a way of giving physical (operational) meaning to the concepts of
"place" and "co-ordinate system", and show how the observer can define both the
place and time of a remote event. Following Einstein, we consider another
system "in uniform motion of translation relatively to the former". Without
assuming "the properties of homogeneity which we attribute to space and time",
we show that the definitions of space and time in the two systems are linearly
related. We deduce some novel consequences of our approach regarding
faster-than-light observers and particles, "one-way" and "two-way" velocities
of light, symmetry, the "group property" of inertial reference frames, length
contraction and time dilatation, and the "twin paradox". Finally, we point out
a flaw in Einstein's argument in the "Electrodynamical Part" of his paper and
show that the Lorentz force formula and Einstein's formula for transformation
of field quantities are mutually consistent. We show that for faster-than-light
bodies, a simple modification of Planck's formula for mass suffices. (Except
for the reference to Planck's formula, we restrict ourselves to Physics of
1905.)Comment: 55 pages, 4 figures, accepted for publication in "Foundations of
Physics
Galactic interstellar filaments as probed by LOFAR and Planck
Recent Low Frequency Array (LOFAR) observations at 115-175 MHz of a field at
medium Galactic latitudes (centered at the bright quasar 3C196) have shown
striking filamentary structures in polarization that extend over more than 4
degrees across the sky. In addition, the Planck satellite has released full sky
maps of the dust emission in polarization at 353GHz. The LOFAR data resolve
Faraday structures along the line of sight, whereas the Planck dust
polarization maps probe the orientation of the sky projected magnetic field
component. Hence, no apparent correlation between the two is expected. Here we
report a surprising, yet clear, correlation between the filamentary structures,
detected with LOFAR, and the magnetic field orientation, probed by the Planck
satellite. This finding points to a common, yet unclear, physical origin of the
two measurements in this specific area in the sky. A number of follow-up multi-
frequency studies are proposed to shed light on this unexpected finding.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
Dark gas in the solar neighnorhood from extinction data
When modeling infrared or gamma-ray data as a linear combination of observed
gas tracers, excess emission has been detected compared to expectations from
known neutral and atomic gas as traced by HI and CO measurements, respectively.
This excess might correspond to an additional gas component. This so-called
"dark gas" (DG) has been observed in our Galaxy, as well as the Magellanic
Clouds. For the first time, we investigate the correlation between visible
extinction (Av) data and gas tracers on large scales in the solar neighborhood.
Our work focuses on both the solar neighborhood (|b|>10\degr), and the inner
and outer Galaxy, as well as on four individual regions: Taurus, Orion,
Cepheus-Polaris and Aquila-Ophiuchus. Thanks to the recent production of an
all-sky Av map, we first perform the correlation between Av and both HI and CO
emission over the most diffuse regions, to derive the optimal (Av/NH)^(ref)
ratio. We then iterate the analysis over the entire regions to estimate the
CO-to-H2 conversion factor as well as the DG mass fraction. The average
extinction to gas column-density ratio in the solar neighborhood is found to be
(Av/NH)^(ref)=6.53 10^(-22) mag. cm^2, with significant differences between the
inner and outer Galaxy. We derive an average XCO value of 1.67 10^(20) H2
cm^(-2)/(K km s^(-1)). In the solar neighborhood, the gas mass in the dark
component is found to be 19% relative to that in the atomic component and
164 relative to the one traced by CO. These results are compatible with the
recent analysis using Planck data within the uncertainties of our measurements.
We estimate the ratio of dark gas to total molecular gas to be 0.62 in the
solar neighborhood. The HI-to-H2 and H2-to-CO transitions appear for Av
0.2 mag and Av mag, respectively, in agreement with
theoretical models of dark-H2 gas.Comment: 9 pages, 4 figures, 1 table. Accepted for publication in A&A (in
press
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